And like described within the link i posted, you can set a lower resolution in “Game Settings, Per System Advanced Settings, Gamecube, VideoMode”.
You can set for example 640×480 (or something else that is low, which you can choose from in the list.
It may depend on the TV/Monitor you are using, which resolutions you can choose from).
The only issue with this method: This will not only change the internal emulator resolution, but also the displayed resolution of your TV/Monitor, which can have some weird side effects, depending on how your TV handles lower resolutions (one of my TVs for example automatically sets another refresh mode (75Hz), when i lower the resolution in this setting. And then the colors also look different (don’t know why my TV behaves like this and i can not change it).